Proportional valve arrangements, as are used, for example, as servo valve arrangements for operating a hydraulic control motor in the form of, e.g. a working cylinder 27 (FIG. 1), may be made up of individual modules D1 through D4. In the form of closing valves that are precontrolled and controlled in a pressure-proportional manner, these modules then form four controllable throttle devices D1, D2, D3, D4 of a hydraulic bridge circuit represented in FIG. 1.
FIG. 1 shows the basic circuit diagram of the so-called open-center variant (variant according to the open-principal) of such a servo valve arrangement, in the neutral position, in which all four throttle devices are open, so that a fluid conveyed by a pump P from a tank T may flow back nearly unhindered through the throttle devices to tank T. Lines LA and LB run from points A and B, respectively, to working chambers a and b, respectively, of working cylinder 27, which are separated from each other by a working piston AK. In the neutral position shown, the pressure in the two working chambers a, b is the same, so that working piston AK remains at rest.
Not all modules D1 through D4 must be controllable. In order to be able to control the movement of the working piston, it is basically sufficient to have a series connection of two throttle devices D1 and D4, of which one must be controllable. In this context, the working piston may be provided a spring, which pushes it in a direction, or working chamber b may be kept at another controlled or constant pressure, whose magnitude is between the pump pressure and the pressure of the tank (mostly atmospheric pressure).